The reduced pressure or vacuum naturally lowers the boiling point of the liquid, and vacuum evaporation is therefore used to advantage in all cases where high boiling temperatures would injure the produce (sugars) or in cases where the boiling point of the concentrated liquor is so high that even direct steam would not give sufficient temperature difference to cause boiling (caustic soda). Where large quantities of cold liquid are to be handled, the low boiling point gives the additional advantage that the liquid has not to be heated to the higher boiling point due to atmospheric pressure.
Evaporation in Multiple Effect—This process may be carried out under pressure or under vacuum. Multiple effects working under pressure are very rare, and are only used for water distillation. Vacuum evaporation in nmilt.inia effect is used extensively for all kinds of solutions, and forms the most important branch of this industry.
Weak _^ Liquor
FIG. 2.—Diagrammatic view of multiple-effect evaporator.
In multiple-effect evaporating system, a number of evaporators are connected in series so that the vapor produced by the steam in the first unit is introduced into the steam chest of the second effect; the vapor of this evaporator is again passed into the steam chest of the third effect, and so on as shown in Fig. 2.
The latent heat of the steam entering the first effect passes through the heating surface and causes boiling of the liquid. The resultant vapor containing practically all the latent heat of the steam will give off this latent heat again to the liquor in the second effect, and so on. The actual work is done by the transfer of the latent heat of the original steam through the heating surface from one evaporator to the next until it is finally condensed in the condenser attached to the last effect. Theoretically, there is almost no limit to the number of times that this latent heat may be transferred; practically, however, the limits are given by the fact that it takes a certain temperature difference between the vapor on one side of the heating surface and the liquid on the other to cause boiling and produce again vapor. For plain water and very weak solutions, this limit of temperature fall is 10°F., and for other liquor, from 20 to 30°F. In the first case, the limit is a decituple effect, and for ordinary conditions a triple or quadruple effect.